Spray-formed steel tooling is fabricated by first creating a pattern of the desired tool using a free form fabrication technique. The pattern is then used to create a ceramic mold that is the inverse of the tool to be produced. The resulting ceramic mold is the receptor into which metal is sprayed to form a deposit in the shape of the desired tool.
At least one spray forming process is wire-arc spraying. In wire-arc spraying, electric current is carried by two electrically conductive, consumable wires with an electric arc forming between the wire tips. A high-velocity gas jet blowing from behind the moving wires strips away the molten metal that continuously forms as the wires are melted by the electric arc. The high-velocity gas jet breaks up or atomizes the molten metal into finer particles in order to create a fine distribution of molten metal droplets. The atomizing gas then accelerates the particles away from the electrode tips to the substrate surface where the molten particles impact the substrate to incrementally form a deposit in the shape of the tool.
The spraying process results in a fine microstructure that produces improved strength, toughness and ductility over metals that are produced by typical casting methods. Regulating the temperature of the deposit controls residual stresses. Thus, geometric accuracy and low residual stresses in the as-deposited tool are achieved.
The completed tool is backed up with an aluminum epoxy backfill material. The completed tool is then mounted and used to produce parts just like any other stamping, die casting, or molding process. There is concern about fatigue life because the porosity of the tool produced is five percent or more, but preliminary estimates of the fatigue life of these parts under load is 10,000 or more cycles which is acceptable for prototype tool applications.
Spray-forming of steel is used for rapid production of prototype and production steel tooling at considerable cost savings over the conventional production by machining and heat treatment of steel tooling for such applications as injection molding and stamping. Typically small sets of prototype permanent tooling can take from four to five months to fabricate, while large prototype tooling can take up to one year to fabricate. Spray-formed steel tooling has been produced in one month at costs substantially less than for permanent tooling.
Such spray-formed steel prototype tooling however has a limited lifetime than its conventional machined steel counterpart, due, in part to the more granular nature of the thermally-sprayed steel, and generally lower overall strength of the sprayed material. Thus, spray-formed steel tooling may be expected to incur a greater need for repair during its useful lifetime and subsequent extension of lifetime through advanced surface repair processes. Typical tool repair involves arc welding new material on the tool, then machining and polishing the material to match the tool. However, the non-homogeneous nature of thermal spray-formed steel does not allow for the practice of common welding techniques used on tool steel.